Laser processing, such as laser cutting and laser welding, is widely used for processing a variety of materials. The lasers typically used for laser processing are CO2 lasers or Nd—YAG lasers.
As an example are lasers, such as CO2 lasers or Nd—YAG lasers widely used for laser cutting virtually all kinds of material, irrespective if they are electrically conducting or non-conducting, hard or soft. A typical set-up for laser cutting comprises a laser, beam guidance and focussing optics, and means for moving the laser beam and the work piece with respect to each other. In a melt-and-blow-type process the laser cutting is aided by an assist gas jet through a nozzle which is concentrically arranged around the laser beam in order to blow the molten material out of the kerf.
However, the CO2 laser has the disadvantage that the beam is strongly absorbed in plumes of metal vapour, readily ionizing molecules in the plume and thus generating even more absorbing plasma.
One major problem of the Nd—YAG laser is that it is difficult to manufacture a high power laser having a good beam quality as for example expressed by a low M2 value.
Further, disc lasers and fibre lasers are known having a very low beam parameter product (BPP). However, power of known single mode fibre lasers is limited to less than 1000 W.
Cut depth and processing speed are determined by the energy absorbed by the work piece and the capability of removing the molten material from the kerf. In known laser cutting processes these challenges are met by increasing the laser power and the pressure of the assist gas. A number of limitations are encountered by this procedure.
One limitation is the formation of an excessive vaporisation plume within and out of the kerf, obscuring the optical path of the processing beam on its way to the cut-front. This is particularly the case for processes based on keyhole-formation. Absorption of laser energy by the vapour will lead to ionisation of molecules in the vapour generating even more absorbing plasma.
A further limitation of this procedure is that the assist gas suffers a pressure drop in the nozzle itself, in the region between the nozzle and the work piece surface, and on its way into and through the kerf. In deep kerfs it is therefore difficult to achieve an assist gas pressure at the bottom of the kerf that is sufficient to effectively remove the material molten by a melting beam. These limitations affect for example the cut quality, leading to defects, such as rough cutting edges or burrs. Some processes, for example laser cutting using CO2-lasers may give a good cut-quality, however at a low cutting speed.
JP 2004 358 521 discloses a laser process, wherein a number of secondary beams is superimposed to a primary beam with substantially the same optical axis and focused to different levels within the work piece. The superimposed beams melt and evaporate material from the work piece, thereby forming a single keyhole. The work piece may be processed along a curve by displacing the work piece and the laser beams with respect to each other. By applying an assist gas the melt generated by the laser beams may be ejected from the work piece, thereby cutting the work piece along the curve.
U.S. Pat. No. 4,870,244 discloses a method for laser drilling or cutting using a first beam for partially melting a work piece and an accurately timed laser pulse from a Q-switched laser directed to the same spot as the first beam in order to create a detonation shock wave in the melt, thereby forcing at least part of the melt out of the processing region and away from the work piece. The disclosed method does not allow for viable high-speed cutting. The short and intense laser pulses from the Q-switched laser and the generated detonation wave tend to lead to substantial turbulences in the melt, strongly affecting the cutting speed and kerf quality. Furthermore, a system implementing the method will suffer from system complexity, amongst others requiring a Q-switched laser and a precise control device for controlling the firing of the laser pulse from the Q-switched laser.